The post-translational modification of histone proteins is now established as an important mechanism for regulating gene expression in eukaryotic cells. However, a structural and mechanistic understanding of how the histone modification enzymes function on their nucleosome substrate is lacking. This shortcoming limits interpretation of the wealth of genetic, genomic and biochemical data available, and it hampers development of new therapeutics that target the many chromatin enzymes associated with human diseases including cancer. To address this deficiency, we propose to study how the PRC1 polycomb repressive complex conjugates ubiquitin to histone H2A assembled in nucleosomes. The addition of ubiquitin to histone H2A has significant biomedical consequences: mono-ubiquitylation of histone H2A by PRC1 plays a critical role in embryonic stem cell maintenance, while mono-ubiquitylation of histone H2A by BRCA1 may mediate this breast cancer protein's tumor suppressor function. This proposal focuses on the following two specific aims: 1. Determine structure of the PRC1 ubiquitylation module on the nucleosome. We propose to crystallize the E2/E3 ubiquitylation complex of UbcH5c/Ring1/Bmi1 in complex with the nucleosome core particle and to determine the atomic structure of the complex by X-ray crystallography. 2. Elucidate mechanism for PRC1 module ubiquitylation of nucleosomes. The crystallographic studies will be complemented by biochemical studies to analyze the roles of the UbcH5c E2 ubiquitin-conjugating and the Ring1b/Bmi1 E3 ubiquitin ligase proteins in ubiquitylating nucleosomal H2A.